1. Field of the Invention
The field of the art to which this invention pertains is that of hydrocarbon processing. In particular, this invention relates to an improved process for the hydrogen fluoride-catalyzed alkylation of hydrocarbonaceous compounds. This invention specifically relates to an improved apparatus for conducting hydrogen fluoride-catalyzed alkylation reactions.
2. PRIOR Art
Various catalytic compounds have been used in the art for the promotion of alkylation of hydrocarbon species. Of particular current interest are the products of alkylation of isoparaffins with olefins to produce antiknock motor fuels and the alkylation of aromatics with olefins to produce precursors of biodegradable detergent products. Catalysts which contain hydrogen fluoride as their essential active ingredient possess considerable advantage over other possible catalytic agents such as a metal halide with a halogen halide promoter, phosphoric acid, and sulfuric acid. For example, when using a hydrogen fluoride catalyst the alkylation reaction may be conducted at a substantially higher reaction rate than with sulfuric acid, while, at the same time producing less undesirable side reactions. The used hydrogen fluoride catalyst may be withdrawn from a processing zone and conveniently regenerated to produce purified hydrogen fluoride, which can be returned to the processing zone for reuse. A spent hydrogen fluoride catalyst commonly contains water and heavy hydrocarbonaceous polymers as well as hydrogen fluoride. Universal practice is to regenerate this spent acid by distillation, or more commonly by fractionation, of the spent acid to yield a stream of contaminants and a stream of relatively pure and anhydrous hydrogen fluoride. The return of this high-purity hydrogen fluoride to the process has presented problems where the process contains multiple reaction zones.
Multiple reaction zone systems are employed in alkylating plants where their use can be justified by the attainment of increased product octane, increased operating flexibility, or increased plant production capacity. Such a plant may have twin reaction zones or even higher multiples of identical reaction zones arranged in a single stage, parallel flow, configuration. The common objective of such a design is to provide higher production capacity in an existing plant or to compensate for size limitations in the ironmongery and unit operations equipment which may be purchased or may be convenient of fabrication. Such a design may also offer the advantage of increased operating flexibility. For example, if it is unavoidable that a plant will receive a varying rate of feed a designer may elect to provide multiple parallel reaction zones whereby the bulk of the processing equipment may be kept in operation and individual reaction zones may be placed in operation or withdrawn from operation as is necessary to compensate for the variation in incoming feed rates. This is a most attractive alternative to cessation of process operation when insufficient incoming feed would cause poor operation of a single, large reaction zone.
Another type of multiple reaction zone system which is in current use in the multiple-stage, series-flow, configuration of reaction zones. It is well known in the art that often multiple consecutive stages of reaction of feeds will produce a superior product to that produced in a single stage of reaction. In such cases the feeds pass through two or more reaction zones in series, contacting an alkylation catalyst in each zone. r
A problem shared by prior art multiple reaction zone alkylation processes utilizing hydrogen fluoride arises in the maintenance of the desired quantities and strengths of the acid catalysts within the separate reaction zones. Catalyst is lost from a reaction zone by virtue of its solution within or entrainment by an exiting stream of reaction products. The strength of acid catalyst within a reaction zone will gradually decrease during operation due to the accumulation therein of excess water and polymerized hydrocarbons. Because the rate of decrease of acid quantity or acid strength among the multiple reaction zones may differ quite radically, periodic corrective action is needed to be taken to insure that each zone maintains a sufficient quantity of the proper strength acid catalyst therein. This is particularly important in prior art designs of alkylation processes in which spent catalyst is removed from one reaction zone, regenerated, and returned to a different reaction zone. In such processes it is not uncommon to find that the continuous entry of relatively pure hydrogen fluoride into the receiving reaction zone contributes to a situation of excessive strength of the acid in that zone. As is well known in the art, hydrogen fluoride catalyst of excessive strength, that is, stronger than about 98 wt. % HF, will cause a reduction in process efficiency. When acid strength is increased above the optimum value in a motor fuel alkylation unit, for instance, an increase in the end point of the alkylate boiling range is noted as well as a decrease in the alkylate's octane number. The transfer of acid catalyst from one reaction zone to another for the purpose of controlling acid strength is not new in the art, however it has traditionally been effected through the use of transfer pumps which do not operate continuously, as a rule. Because hydrogen fluoride is an extremely corrosive material when combined with water, as is the case with hydrogen fluoride alkylation catalysts, the transfer pumps traditionally used have been constructed of highly special and very expensive alloy formulations. However, even with the use of these special alloys in the construction of the transfer pumps the maintenance problems involved with their use can be enormous. This has been the case primarily because conventionally used pumps have been of the reciprocating or centrifugal variety, and such pumps contain moving parts which slide across each other while in contact with acid catalyst. This continuous sliding provides, with every sliding action, a fresh metal surface for attack by hydrogen fluoride. In other words, a protective film of oxide is not possible of formation.
Our invention presents an improvement over the prior art by provision of a combination of the well known principal of education with hydrogen fluoride alkylation processes to substantially eliminate the processing problems associated with acid control through the use of pumps in the transfer of acid catalyst from reaction zone to reaction zone.